Variable frequency selective circuits



NOV. 28, 1950 4. HOLLAND ETAL 2,531,434

VARIABLE FREQUENCY SELECTIVE CIRCUITS Filed July 17, 1947 2 Sheet s- Sheet 1 F/Gi F/G.2.

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Nov. 28, 1950 J. D. HOLLAND ETAL 2,531,434

VARIABLE FREQUENCY SELECTIVE CIRCUITS Q I /n venfors frkflfz W Patented Nov. 28, 1950 VARIABLE FREQUENCY SELECTIVE CIRCUITS John Douglas Hollandand Duncan Dove Robinson, London, England, assignors to International Standard Electric Corporation,

York, N. Y.

New

Application July 17, 1947, 'SerialNo. 761,614 In Great Britain July 13, 1946" Section 1, Public Law 690., August 8, 1946 Patent expires July 13, 1966 6 Claims.

The present invention relates to frequency selective circuits in which the resonant frequency is made variable over given range of frequencies and is particularly directed in such a circuit to the control of effectsofsmall variations in the values of the circuit elements upon the resonant frequency as determined by the adjustment of a tuning element. These small circuit element changes may be deliberately" made by the useras in the case of vernier tuning adjustments-or may be 'unwanted -as in the case of ambient temperature effects. Although attention will be directed to cases where ambient temperature or Vernier tuning control are the parameters effecting such circuit elements, it is conceivable that a variable frequency tuned circuit may be used in circumstances. where some other parameter, such as mechanical strain, may give rise to changes in certain-of the circuit elements values. In'order, as far as possible, to use a generic term, the changes will be considered as proportioned to variation coefiicients of the values of the elements in question with respect to a given parameter.

In a fixed .frequency'tuned circuit itis usually a fairly straightforward matter to regulate the resonant frequency variation of the whole circuit in. terms of. variation coefficients of the circuit components. When the circuit may be tuned over. a band of frequencies, however, regulation is not so simple. In the case of temperature compensation there have been proposals to achieve compensation by means such as designing the tuning condenser to have a diiferent temperature coefiicient according to the setting of the condenser. In the same way difiiculty arises if it be desired to arrangetha t the'deviation obtained by a Vernier tuning control shall obey a definite law throughout the frequency band. We have found, however, that by the simple choice of component values and of the variation coefiicient thereof,it ispossible ina circuit comprising say an inductance and three condensers, to predements are adjusted so as to cause the overall proportional frequency deviation,

of the resonant frequency, caused by changes in that parameter to which said variation coefficients are related, to be substantially predetermined for two settings ofsaidvariable reactance element. The invention further provides such a circuit in which said predetermined values of i are the same and in which the two settings of said variable reactance elements are so chosen that do) I:

varies a minimum amount from said predetermined value of within said given range.

In order that the invention may be more fully understood, an analysis :of certain circuit configurations will now be given and reference made to the accompanying drawings which are for the most part purely 'explanatory'in nature andwhich it is, therefore, not proposed to list figure by figure.

In its simplest form a parallel tuned circuit may be reduced to the configuration of Fig. 1, in which L is the tuning inductance and CT the total effective tuning capacity. Then for resonance of frequency We shall consider first the case where it is desirable that may be as small as possible, as in the case where we are dealing with temperature variation. This hypothesis requires that over the range of variation.

In general it is a matter of considerable difficulty to predetermine temperature coefficients of variable condensers. Furthermore, Or will inevitably include a minimum capacity Co, due to wiring and valve capacities and the like, directly shunting L; hence we cannot in practice have a circuit simpler than that of Fig. 2 in which Co in parallel with a variable condenser C2 constitute C'r.

Before proceeding further, it is well to establish the value of T for two condensers, each having difierent coefficients, in parallel and in series. Referring to Fig. 3 and Fig. 4, the two capacities are denoted by :00 and g0, 93 and y being numerical multipliers, and it is assumed that the respective coefiicients of these condensers are A and t. Then for the parallel case T w+ J and for condensers in series my w+l/ (3b) M +Ml x-i-y Returning now to the circuit of Fig. 2, let us assume that C2=nCo, where n is variable to change the resonant frequency, and that the coefllcient, say of temperature, of Co and C1 are and ,3 respectively. Then Combining Equations 2 and 4 it will be seen that in order to obtain compensation we must have In particular, if 7La 1, and =a, there will be compensation at the highest frequency within the range. This means, in practice, that we can build out Co to a suitable value by means of fixed condensers employing a dielectric having a negative temperature coefiicient.

In order that more complete compensation may be obtained another controllable circuit element 4 must be introduced. It is obvious that no further improvement is to be gained by adding elements in parallel, except possibly in order to obtain the required value of more readily. The other alternative, is to insert a third condenser in series with C2. In Fig. 5, C3, of value aCo and having a coefficient 7, represents this additional element. It should be observed that the configuration shown in Fig. 5 is that usually adopted for the local oscillator tank circuit in superheterodyne receivers When it is desired to gang together the signal frequency tuning condensers with the oscillator tuning condenser C2, and it is therefore particularly aposite.

From consideration of Equations 3, or otherwise, the values of CT and for the circuit of Fig. 5 are found to be an i m] n (l+a)+un(2+a)+a so that the compensation condition becomes For complete compensation for all frequencies vanish at two arbitrary values of n, and ,1 2. by eliminating and in Equation 9 we obtain The general shape of the curve is shown in Fig. 6. The curve has asymptotes n=-a, n=b, and y=l. In the region of posi-' tive values of n, there is a minimum at (no, Hence, provided |yol 1 there will be two points on the curve to the right of n=-b having ordinates go and abscissa m, m say. Thus, for a given deviation of :L-yo, n may vary between 111' and n2. Conversely, if m and m be given limits of variation of n, y will be as small as possible within this range if the zeros are chosen so that the ordinates at m and m are each equal in magnitude but opposite in sign to the resulting value of ya.

Since ass-mar:

. As a practical example, let us take the case of a local oscillator tank circuit to cover a signal frequency range of 1 to 2 mc./s, with an intermediate frequency of 580 kc-./s. The circuit components are assumed to have the following values:

Equation 12 may be written in the form dw g- A )1! where A=0.918 for the present example.

The resulting curve for Ag is shown in Fig. 7, the range to be covered being indicated by AA. The maximum variation of Ag is :5.63 10- Although we have been considering the case of a circuit with a variable tuning condenser, an analogous circuit may be derived in which a variable inductance is used. The circuit of Fig. 8, in which a is now the coefficient of C, {3 of the variable inductance nLo, of L and 'y of aLo, is exactly equivalent in analysis to that of Fig. 5.

The case where a given adjustment of a vernier tuning control is desired to cause a proportional frequency deviation which is constant throughout the main tuning range of the circuit is very similar to the above. a and 3 become zero, while 41 and 7 are chosen so that instead of being zero, sends to a constant value K, say, between m and n2.

Insofar as the previous analysis is concerned, this change ismerely equivalent to substituting 2K for a and putting 18:0, combined with a shiftof the origin of co-ordinates efiected by writing in place of wherever the latter appears in the analysis from Equation 8 onwards. The method of calculation being the same, itremains to utilize the re-- sults in a practicable manner. Since and 'y are not, in general, equal,. modificationv of the circuit of Fig. 5; is called for if it is desired. to

use similar trimmer condensers in both C3 and Co. It will be found that, for the general case where the mean values of Co, and C3 are given,

and v and 5 are due'to equal increments in similartrimmer condenser capacities, the simplest arrangement is. to replace each of Co and Ca by thecapacity network shown in Fig. 9, the values of a: and 1,! being different for the two networks, but, the mean capacity C and its variation coefficient A, say, being the same in each case.

For the network of Fig. 9 we have in which X is the variation coefficient of the trimmer condenser C.

In terms of CT, (Jr, C and l, the values of :c and are given by i 92.2. l+y C x yo 0 i 18) and the conversion is possible provided One particular arrangement of a resonant circuitto whichthe above principles maybe appliedis shown in Fig. 10. C2 is the main tuning condenser and C1 is adjusted to take up the effects of variation in the wiring or valve capacities of the external circuit. Cs and C8 are the trimmer condensers ganged together and to a vernier tuning control. The combination consisting of C1, C4, C5 and C6, together with any external circuit capacity comprises the C0 of Fig. 5 and the combination of C7, C8 and C9 comprise C3.

It will be evident that provided the trimmer condensers of Fig. 10 have very small temperature c0efiicientssay less than 5x10- ,u,uf.//L;.Lf./C C4 and C1 may have their temperature coefiicients adjusted so as to provide temperature compensation in accordance with the present invention.

An examination of the condenser network of Fig. 9 and of Equations 17 and 18 shows that when m 1, CT is practically independent of y so that the proportionality between 01 and A depends directly on the values of Hence, in Fig. 10, C5 and C9 could be made variable and ganged together to vary the amount of deviation provided by the vernier tuning control Without appreciably effecting the constancy thereof throughout the tuning range covered by the main tuning condenser C2. One application of such a modification is in connection with a cathode ray oscillograph used as a spectrometer. The resonant circuit of Fig. 10 provides the tuned circuit of an oscillator whose output is fed to a circuit whose frequency response curve, for example, it is required to reproduce on the oscillograph screen. Cs and Ca may be rotating condensers, or might well represent electronic react ance circuits, varying in step with the sweep circuit of the oscillograph so as to provide a frequency modulated output to the circuit under test. In this case the condensers C5 and C9 could be varied together to open out or close up the width of the display on the oscillograph screen.

What is claimed is:

1'. A frequency selective circuit comprising a variable reactance, a first reactance connected to said variable reactance, second and third reactances connected in parallel with one another and in parallel with said connection of said variable and first reactances, all said reactances having temperature coeflicients equal in size and said third reactance only having a temperature 00- efiicient opposite in sign to that of said other react-ances.

2. A frequency selective circuit comprisin a variable condenser, a first fixed condenser connected in series therewith, an inductance and a second fixed condenser connected in parallel with one another and in parallel with said series connection of said variable and first fixed condenser,

said condensers having equal temperature coeflicients and said inductance having a temperature coefficient equal in size and opposite in sign to said first mentioned temperature coefiicients.

3. A frequency selective circuit according to claim 2, in which each of said fixed condensers comprises a group of condensers of like value having different temperature coefiicients and adapted to be connected in series or parallel relationships so that the most nearly correct value of the temperature coefficient of the group may be obtained without alteration of the total capacity of the group.

4. A frequency selective circuit according to claim 3 in which some of said condensers in said groups comprise subsidiary variable condensers ganged to a vernier tuning control whereby the proportional variation ofcapacity of each 10f said' groups due to a given change of said vernier control is adjusted by choice of suitable values of associated fixed condensers in each said group to give a predetermined value of frequency deviation at two settings of said first-mentioned variable condenser.

5. A frequency selective circuit according to claim 4 in which the temperature coefiicient of said subsidiary variable condensers are made .very small compared to the coeflicients of the other condensers of each of said groups whereby adjustment of the temperature coeflicients of said fixed condensers in each of said groups will provide a group temperature coefficient of the desired value.

6. A frequency selective circuit comprising a variable inductance, a first fixed inductance connected in series therewith, and a condenser and a second fixed inductance connected in parallel with one another and in parallel with said series connection of said variable and first fixed inductances, said inductances having equal temperature coeflicients and said condenser having atern- I perature coefificient equal in size and opposite in sign to said first mentioned temperature coefficients.

JOHN DOUGLAS HOLLAND. DUNCAN DOVE ROBINSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,005,772 Chireix June 25, 1935 2,231,389 Koffyberg Feb. 11, 1941 2,250,090 Buschbeck July 22, 1941 40 2,310,797 Lea Feb. 9, 1943 

